The present invention relates to image sensors, and more particularly, towards an image sensor that has taller micro-lenses in the outer regions of the pixel array.
Image sensors are electronic integrated circuits that can be used to produce still or video images. Solid state image sensors can be either of the charge coupled device (CCD) type or the complimentary metal oxide semiconductor (CMOS) type. In either type of image sensor, a light gathering pixel is formed in a substrate and arranged in a two-dimensional array. Modern image sensors typically contain millions of pixels to provide a high resolution image. Important parts of the image sensor are the color filters and micro-lens structures formed atop of the pixels. The color filters, as the name implies, are operative, in conjunction with signal processing, to provide a color image. The micro-lenses serve to focus the incident light onto the pixels, and thus to improve the fill factor of each pixel.
Conventionally, micro-lenses are formed by spin coating a layer of micro-lens material onto a planarized layer. The micro-lens material is then developed to form cylindrical or other shaped regions that are centered above each pixel. Then, the micro-lens material is heated and reflowed to form a hemispherical micro-lens. FIG. 1 shows a prior art cross-sectional simplified diagram of an image sensor 101 having micro-lenses formed thereon. As seen in FIG. 1, the image sensor includes a plurality of pixels that have light detecting elements 103 formed in the substrate. The light detecting elements 103 may be one of several types, such as a photodiode, a photogate, or other solid state light sensitive element. Formed atop of each pixel is a micro-lens 105. The micro-lens 105 focuses incident light onto the light detecting elements 103. Moreover, in the region between the light detecting elements 103 and the micro-lens 105, denoted by reference numeral 107, there are various intervening layers that would typically include the color filter layers and various metal conducting lines. These components are excluded from the diagram in order to simplify the explanation herein and not to obscure the invention.
In the prior art, the formation of the micro-lenses is controlled such that the shape of the micro-lenses exhibits uniformity throughout all of the pixels of the image sensor. However, the applicant has observed a variation in the amount of light captured by the pixels, with the pixels near the center of the image sensor collecting more light than the pixels in the periphery of the image sensor.
FIG. 3 illustrates one reason why this occurs. In FIG. 3, an image sensor 101 typically works in conjunction with an imaging lens 204 to capture an image. The imaging lens 204 takes incident light and transmits it to the image sensor 101 as shown in FIG. 2. As seen, for those pixels that are located in the center of the image sensor 101, the incident light from the imaging lens 204 is focused correctly to the light detecting elements. However, for those pixels that are located at the outer regions of the image sensor 101, the incident light from the imaging lens 204 is not perpendicular to the micro-lens, thereby causing the focused incident light to be misaligned to the light detecting element. This in turn results in relatively less light being captured by the light detecting element. This is referred to herein as the xe2x80x9cdark cornerxe2x80x9d phenomena.